When did I know? It's actually a bit of a complicated question. I think that if I had grown up in a more gendered household, I would have known earlier, but my home was very egalitarian - Everyone did housework, everyone did yard work, everyone did heavy lifting. My sister taught me how to drive stick, my Army brother taught me how to sew on a button.
So for me, it wasn't until I tried to start tackling my depression. I'd been depressed since high school, and at first I thought "well, you're a teenager, of course you're depressed," so I did nothing and just tried to wait it out. But then I did a three year college program, doing what I love (music), and was still depressed. Then I went to university after that college program, and I was still depressed. So I started seeing a therapist irregularly, and got onto some antidepressants.
Here's the thing about antidepressants that most people don't get. Their job isn't to make you happy, or to numb you. their job is to get you to a place where you can deal with your shit. It turned out my "shit" was being trans.
I... didn't want that to be true. So I grew a beard, and tried to be "manly". A few years later, at the end of my bachelor's degree, I had a moment where I realized that I hadn't looked at myself in the mirror in 6 months - This is more difficult than it sounds, the average person at the very least looks in the mirror when they wash their hands. I looked homeless - Shaggy, unkempt. By december of that year, now in my Master's, I had finally convinced myself to talk to a therapist about trans stuff, and I had come out to a few very close friends.
Is this a similar experience? It's one of many different experiences. There are eight million stories in the naked city, as they say. Mine is just one of them. Some say they knew from their first memories, some live in deep denial until their 80s, some transition in their teens, some don't transition at all.